1. Field of the Invention
The present invention relates to a trip device of a main circuit breaker employed in a selective tripping system in an electric circuit constituted of the main circuit breaker connected to the power supply of the circuit and a branch circuit breaker connected to a load thereof. The selective tripping system causes only the branch circuit breaker to reach a tripped state when a short circuit leading to flow of a large amount of current happens in the load circuit connected to the branch circuit breaker.
2. Description of the Background Art
An electric circuit is generally configured of a main circuit breaker B1 connected to power supply S, a plurality of branch circuit breakers B2, B3 and B4 connected to the main circuit breaker B1 in parallel with each other, and load equipments L2, L3 and L4 controlled and protected by the branch circuit breakers as shown in FIG. 7. These circuit breakers usually have instantaneous trip devices respectively. When an enormous abnormal current such as short-circuit current flows, the instantaneous trip devices trip to open respective switching contacts and thus break the abnormal current.
In such an electric circuit, if a short-circuit accident occurs at a point X1 between main circuit breaker B1 and the branch circuit breakers, only the instantaneous trip device of main circuit breaker B1 operates to instantaneously break the abnormal current. As a result, main circuit breaker B1 reaches a tripped state so that feeding of electric power to all branch circuits is inevitably stopped.
If a short-circuit accident occurs at a point X2 on the side of the load connected to branch circuit breaker B2, for example, the short-circuit current flows through main circuit breaker B1 and branch circuit breaker B2. Consequently, not only branch circuit breaker B2 but also main circuit breaker B1 is set into the tripped state. Feeding of electric power to branch circuit breakers B3 and B4 where no accident occurs is accordingly stopped to interrupt the operations of load equipments L3 and L4.
An accident in one branch circuit thus interrupts an operation of a load equipment connected to another normal branch circuit. Such a state should generally be avoided as much as possible. For this reason, the selective tripping system is employed in order to keep supply of electric power to branch circuit breakers B3 and B4 by causing only branch circuit breaker B2 to trip if an accident happens at point X2 shown in FIG. 7.
Regarding the selective tripping system, an improvement in a conventional scheme is disclosed (for example, in Japanese Patent Laying-Open No.3-101023). A main circuit breaker B1 used in the conventional scheme has a short-time tripping device instead of the instantaneous trip device to prevent main circuit breaker B1 from tripping for a short period of time during which a branch circuit breaker B2 breaks the abnormal current when an accident occurs at point X2. Such a circuit configuration can endure the abnormal current at point X1 shown in FIG. 7 for a short period of time as described above.
In main circuit breaker B1 of the conventional selective tripping system, the operating time of the short-time tripping device should be made as short as possible in order to break the abnormal current at point X1 as soon as possible. In order to meet this requirement, an electronic trip device having a relatively constant operating time is employed as the shorttime tripping device. However, a problem of the electronic trip device is the high cost and large size of the device.